1. Field of the Invention
The invention is related to the design of tunable inductor coils used in inductance-capacitance (LC) tank voltage controlled oscillators (VCO) and phase-locked-loop (PLL) circuits.
2. Description of Related Art
Phase noise and timing jitter are of importance in the design of oscillators and frequency synthesizers. A known method to design high performance clock sources is to use LC-tank oscillators, which are able to achieve better phase noise performance than ring oscillators because of the higher quality factor (Q-factor) of the LC-tank. In wireline communication however where often multiple standards with partially widely spaced frequency ranges have to be covered, the application of LC VCOs is rather difficult since the tuning range of LC-tank oscillators is limited to about 30% if regular LC-tank topologies are used.
It would therefore be desirable to increase the tuning range of LC-tanks to extend the frequency ranges of LC VCOs.
One way of increasing the tuning range of LC-tanks is to use more varactor (variable capacitor) banks, which would increase the variation of the capacitance in the LC tank. This method is however practically limited by the capacitive and resistive parasitics that occur when increasing the number of varactor banks. The additional capacitive parasitics would decrease the Cmax/Cmin-ratio of the actual tuning varactor and the resistive parasitics would degrade the quality factor disproportionately because of the longer wiring traces required to connect the different varactor banks to the inductor coil and the other devices of the LC VCO.
Moreover the peak of the quality factor of the varactor bank is at relatively low frequencies because the varactor impedance is inversely proportional to the frequency (Zvar˜½πfC) and the varactor quality factor decreases with increasing frequency. This is in contrast to the inductor where the peak of the quality factor is typically at higher frequencies because the inductor impedance changes proportionally to the frequency (Zind˜2πfL). At lower frequencies the overall quality factor of the LC-tank given by
      1          Q      LC_tank        =                    1                  Q          Varactor                    +              1                  Q          Inductor                      =                          ⁢                  (                      Z            LC_tank                    )                                    ⁢                  (                      Z            LC_tank                    )                    is dominated by the quality factor of the inductor whereas at higher frequencies QLC—tank is mainly determined by the quality factor of the varactor. This implies that for the operation of a LC VCO at higher frequencies the tuning or band selection should preferably be performed with a tunable or switchable inductor instead of a set of switchable varactor banks as this is typically implemented in state-of-the-art LC oscillators.
However, switching of inductor coils does degrade the quality factor because of the resistive losses of the closed switches. But if implemented appropriately the degradation of the quality factor is not that high so that the resulting quality factor of the switched inductor coil would not become smaller than the quality factor of the varactor bank at the frequency of interest. This statement is valid up to a certain frequency. At extremely high frequencies the LC VCO cannot be operated anymore for instance because of frequency divider limitations in a PLL circuit. Additionally, other parasitics like bridging capacitances across the inductors come into play that degrade the quality factor. Within a reasonably wide frequency range however the performance of a switchable inductor outperforms that of a switchable varactor bank.
There are two types of switchable inductors: either a switch is series-connected with a self-inductance coil or the switch is located within a secondary isolated coil. Both types of switchable inductors are shown in FIG. 1, wherein P1 and P2 denote outer ports of the switchable inductors.
Referring to FIG. 1, in the switchable coil configuration 01 the switch 02 is series-connected with the self-inductance coil 04. The switch 02 bypasses the inner coil winding 03 of the two-winding-coil 11. If the switch 02 is closed the current flows only in the outer winding 04, which is the primary coil 04 of the two-winding-coil 11, and the inner winding 03, which is the secondary coil 03 of the two-winding-coil 11, is short-circuited and disconnected from the outer winding 04. If the switch 02 is opened, the two-winding-coil 11 is operated like a regular multi-turn coil because both coils 03 and 04 become series-connected.
In the switchable coil configuration 05 shown in FIG. 1(b) the mutual inductance is switched, i.e. the switch 06 is located in the mutual inductance coil 07, which is the secondary coil 07. If the switch 06 in the secondary coil 07 is opened, only the self-inductance of the primary coil 08 contributes to the overall inductance. However if the switch 06 is closed, the current induced in the secondary coil 07 generates a mutual inductance that reduces the self-inductance of the primary coil 08 so that the overall inductance becomes smaller.
The two basic principles of switching an inductor coil can be analyzed based on the lumped element equivalent circuits 09, 10, which are shown in FIGS. 1(c) and 1(d). For equal coil geometries—despite differently connected—and closed switches, it can be shown that:                a) the resulting overall inductance of both switchable coil configurations 01 and 05 is the same but        b) the right-hand side inductor 05 and the switchable coil configuration 05 respectively has a higher quality factor.        
This is caused by the fact that the switch resistance in the closed state does not get transformed one-to-one from the secondary coil 07 to the primary coil 08 in the right-hand side switchable inductor 05. This is in contrast to the left-hand side inductor 01 where the switch 02 is series-connected to the self-inductance coil 04 and therefore the switch resistance fully contributes to the resistive part of the inductor impedance.
From U.S. Pat. No. 6,549,096 it is known to decrease the magnetic field of an inductor by the presence of one or more single loop windings positioned in proximity to the inductor. The single loop windings have open circuits that are selectively closed to magnetically couple the single loop windings to the inductor. Further, it is known to form a switched inductor/varactor tuning circuit by connecting a varactor to the inductor. Thereby different axial and coaxial arrangements of single and multi-loop windings of primary and secondary coils are proposed causing maximum magnetic field reduction when closing a loop switch, thus causing maximum inductance change.
The tuning range obtained by the known inductor/varactor tuning circuit is limited. Further the step size of switched inductance is relatively coarse.